The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the autocorrection of motion artifacts in MR images acquired using phased-array coils.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1, is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy and Gz,) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Object motion during the acquisition of NMR image data produces both blurring and xe2x80x9cghostsxe2x80x9d in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion each view of the NMR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the blurring and ghosting is due to the inconsistent appearance of the object from view to view. Motion that changes the appearance between views such as that produced by a patient moving, by the respiration or the cardiac cycle, or by peristalsis is referred to hereinafter as xe2x80x9cview-to-view motionxe2x80x9d. Motion may also change the amplitude and phase of the NMR signal as it evolves during the pulse sequence and such motion is referred to hereinafter as xe2x80x9cin-view motionxe2x80x9d.
Both blurring and ghosting can be reduced if the data acquisition is synchronized with the functional cycle of the object to reduce view-to-view motion. This method is known as gated NMR scanning, and its objective is to acquire NMR data at the same point during successive functional cycles so that the object xe2x80x9clooksxe2x80x9d the same in each view. The drawback of gating is that NMR data may be acquired only during a small fraction of the objects functional cycle, and even when the shortest acceptable pulse sequence is employed, the gating technique can significantly lengthen the data acquisition.
Another proposed method for eliminating ghost artifacts is disclosed in U.S. Pat. No. 4,567,893, issued on Feb. 4, 1986. This prior patent teaches that the distance in the image between the ghosts and the object being imaged is maximized when the NMR pulse sequence repetition time is an odd multiple of one-fourth of the duration of the periodic signal variation. This can be used to alleviate ghosts due to respiratory motion. While this method, indeed, improves image quality, it does impose a constraint on the NMR pulse sequence repetition time and it often results in a longer total scan time. It also assumes that the motion is periodic.
Yet another method for reducing the undesirable effects due to periodic signal variations is disclosed in U.S. Pat. No. 4,706,026 issued on Nov. 10, 1987 and entitled xe2x80x9cA Method For Reducing Image Artifacts Due To Periodic Variations In NMR Imaging.xe2x80x9d In one embodiment of this method, an assumption is made about the signal variation period (e.g. due, for example, to patient respiration) and the view order is altered from the usual monotonically increasing phase-encoding gradient to a preselected order. For a given signal variation period, a view order is chosen so as to make the NMR signal variation as a function of the phase-encoding amplitude be at a desired frequency. In one embodiment, the view order is selected such that the variation period appears to be equal to the total NMR scan time (low frequency) so that the ghost artifacts are brought as close to the object being imaged as possible. In another embodiment (high frequency), the view order is chosen to make the variation period appear to be as short as possible so as to push the ghost artifacts as far from the object as possible.
This prior method is effective in reducing artifacts, and is in some respects ideal if the variation is rather regular and at a known frequency. On the other hand, the method is not very robust if the assumption made about the motion temporal period does not hold (e.g., because the patient""s breathing pattern changes or is irregular). If this occurs, the method loses some of its effectiveness because the focusing of the ghosts, either as close to the object or as far from the object as possible, becomes blurred. A solution to this problem is disclosed in U.S. Pat. No. 4,663,591 which is entitled xe2x80x9cA Method For Reducing Image Artifacts Due To Periodic Signal Variations in NMR Imaging.xe2x80x9d In this method, the non-monotonic view order is determined as the scan is executed and is responsive to changes in the period so as to produce a desired relationship (low frequency or high frequency) between the signal variations and the gradient parameter. The effectiveness of this method, of course, depends upon the accuracy of the means used to sense the patient motion, and particularly, any variations in the periodicity of that motion.
Yet another method for reducing motion artifacts in NMR images is referred to in the art as xe2x80x9cgradient moment nullingxe2x80x9d. This method requires the addition of gradient pulses to the pulse sequence which cancel, or null, the effect on the NMR signal phase caused by spins moving in the gradients employed for position encoding. Such a solution is disclosed, for example, in U.S. Pat. No. 4,731,583 entitled xe2x80x9cMethod For Reduction of NMR Image Artifacts Due To Flowing Nuclei By Gradient Moment Nullingxe2x80x9d.
The most successful method for correcting MR images for motion artifacts employs navigator signals acquired during the scan. As described in U.S. Pat. No. 4,937,526, such navigator signals are acquired periodically during the scan, and the information in these signals may be used to correct the image data for patient motion. Unfortunately, acquisition of the navigator signals increases the scan time.
More recently an automatic correction method has been developed in which the acquired image is iteratively corrected and measured against a quality metric. Termed xe2x80x9cautocorrectionxe2x80x9d, this method systematically tries different phase correction on the image data and measures the quality of the resulting image. This is done in an iterative process that drives the image quality towards the highest possible quality.
The present invention relates to improvements in the autocorrection approach which make it a practical tool for the correction of clinical images acquired using phase-array coils. Separate k-space image data sets are acquired with each coil element in a phase-array coil. Autocorrection is employed on one of the k-space image data sets, and the resulting phase corrections are made to all of the acquired k-space image data sets. In the alternative, autocorrection may be applied separately to each of the acquired k-space image data sets. In either case, the corrected image data sets are combined to produce a single corrected image.